- Email: [email protected]
In-situ scanning electron microscope observation of electrode reactions related to battery material
Electrochimica Acta 319 (2019) 158e163
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In-situ scanning electron microscope observation of electrode reactions related to battery material Tetsuya Tsuda**, Kei Hosoya, Teruki Sano, Susumu Kuwabata* Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2019 Received in revised form 23 June 2019 Accepted 27 June 2019 Available online 28 June 2019
Scanning electron microscopy (SEM) for the electrode reaction in next-generation battery with ionic liquid (IL) electrolyte, which has both high stability under vacuum and an antistatic nature, is applied to the binder-free high capacity Si anode to observe the morphology change of the Si nanoparticle aggregate active material at different potentials during charge-discharge processes. In the IL electrolyte, 1-ethyl-3-methylimidazolium bis(fluoromethylsulfonyl)amide ([C2mim][FSA]) with 1.0 M lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), the morphology greatly varied at the plateau observed at 0.200 to 0.070 V (vs. Li(I)/Li) in the charge-discharge curve during the first cycle, suggesting that phase transition of a-Si to c-Li13Si4 is a potential problematic process. After the first cycle, the lithiation process initiated at more positive potential. Similar behavior was also recognized in the Li[TFSA]‒tetraglyme solvate IL electrolyte in a 1:1 molar ratio ([Li(G4)][TFSA]). © 2019 Published by Elsevier Ltd.
Keywords: In situ Ionic liquid Battery Si anode Scanning electron microscopy
1. Introduction Several requirements to future battery systems, such as higher capacity and quicker charging, will give serious issues, e.g., morphology change of the electrodes, variation in electrode|electrolyte interphase and undesirable cell deformation. Obviously these issues lead to the deterioration of battery. Although there are some promising future batteries including Li-S rechargeable battery [1e3], all solid Li rechargeable battery [4e6], Mg metal rechargeable battery [7e9] and Al metal rechargeable battery [10e14], these batteries have the above mentioned issues that should be overcome. It means that direct analysis or in-situ observation of the battery reactions will be a key technique to improve the battery performance. In-situ optical microscope observations using airtight electrochemical cells have already been reported to date [15,16]. What is worth noting is that most future batteries employ nonvolatile electrolytes, solid electrolyte and ionic liquid (IL) electrolyte, suggesting that electron microscopy, which requires vacuum condition, is applicable to the battery reaction observation without such airtight cells. ILs are a subset of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Tsuda), kuwabata@chem. eng.osaka-u.ac.jp (S. Kuwabata). https://doi.org/10.1016/j.electacta.2019.06.165 0013-4686/© 2019 Published by Elsevier Ltd.
anhydrous liquid salts that contain only cationic and anionic species and can maintain a liquid state below 373 K. The common features of ILs are negligible vapor pressure, flame resistance, wide electrochemical window and relatively-high ionic conductivity [17,18]. These features are controllable by the combination of cationic and anionic species in the ILs. It is then quite natural to design the IL electrolytes for next-generation batteries by controlling the ionic combination [17e20]. Besides, using IL electrolytes with a negligible vapor pressure enable in-situ and operando analytical techniques conducted along with the electrochemical measurements under vacuum condition. Direct observation of the morphology variations in nextgeneration batteries by in-situ and operando analytical techniques is increasingly important to get new leads for improving the battery performance [21,22]. Several research groups including us have attempted to establish scanning electron microscopy (SEM) techniques for the in-situ and operando observation of electrode reactions in the specially designed electrochemical cells with the IL electrolytes [23e29]. Recently morphology variations in several Si anodes in the full cell type Li-ion battery, which can give useful actual information during the battery operation compared to the half cell type one, have successfully been observed by similar in situ SEM techniques [30e33]. However, there is no detailed information on the variations in the Si anode at different potentials during the charge-discharge processes. In this article, we reveal the morphology variation of the binder free Si nanoparticle aggregate
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
anode by SEM observation at different potentials. For this purpose, the specially designed three-electrode type Li-ion batteries were fabricated using two different IL electrolytes, 1-ethyl-3methylimidazolium bis(fluoromethylsulfonyl)amide ([C2mim] [FSA]) IL with 1.0 M lithium bis(trifluoromethanesulfonyl)amide (Li [TFSA]) and Li[TFSA]‒tetraglyme solvate IL in a 1:1 molar ratio ([Li(G4)][TFSA]). 2. Experimental section In situ SEM observation of the binder-free Si anodes was performed using a three-electrode type full cell with a mesh-type anode, a LiCoO2 composite cathode with an Al foil current collector (3.0 mAh cm2, Hohsen (Japan)), a Ni wire (f 0.2 mm, Nilaco (Japan)), and two sheets of glass microfiber filter separators (GF/A, Whatman (UK)). The Ni wire sandwiched between two glass microfiber filter separators was used as a reference electrode after Li metal electrodeposition on the Ni wire electrode by using LiCoO2 cathode as a counter electrode. The Li metal deposition was conducted at -0.2 mA for 1 h. Its potential was calibrated to LiCoO2 after electrochemical measurements. The theoretical LiCoO2 cathode capacity was enough to observe the Si anode reaction. Binder-free handmade Si anodes were fabricated by an electrophoretic deposition (EPD) method similar to previous paper, but not using acetylene black and citric acid in this study [31]. The solution used for the binder-free Si anode preparation was dry acetonitrile containing 1.0 g L1 of Si nanoparticle aggregates (325 mesh, Rare Metallic Co., LTD. (Japan)). A platinum plate was used as a cathode and a copper mesh was served as an anode. The electrodes were separated by 2 cm in the solution and a voltage of 100 V was kept between them with duration of 60 s. A schematic drawing and photograph of the electrochemical cell for in-situ SEM observation of the Si anode are depicted in Fig. 1. The electrolytes were [C2mim][FSA] (Kanto Chemical Co., Inc. (Japan)) with 1.0 M Li[TFSA] (Morita Chemical Industries Co., Ltd. (Japan)) and [Li(G4)][TFSA]. All Li-ion batteries for in-situ experiments were assembled in an argon-filled glove box (Vacuum Atmospheres Company (USA), OMNI-LAB; H2O, O2 < 1 ppm). Electrochemical in-situ SEM observation system was fabricated by attaching a feed-through terminal into commercially available SEM systems (VE-8800, KEYENCE (Japan); S-3400N, Hitachi (Japan)) to examine the electrode reaction. Electrochemical experiments were controlled with a potentiostat/galvanostat (VersaSTAT 4, Princeton Applied Research (USA)). 3. Results and discussion We have already succeeded in the in-situ SEM observation of
159
different types of Si active materials and their composite negative electrodes for next-generation high capacity battery and revealed that the behavior of the morphology variation is closely related to the battery performance [31,32]. Si anodes with a favorable performance show a good reversible morphology alteration during the charge-discharge process, suggesting that it is important to design the electrode configuration considering the morphology variation after charge and discharge in order to fabricate high performance negative electrodes. However, no detailed information on the relevance between morphology change and charge-discharge potential of Si negative electrode has not yet been reported. Fig. 2a shows charge-discharge curves at the first cycle of the binder-free Si nanoparticle aggregate negative electrode, which were taken by a constant current mode at 0.25C in the cell for electrochemical in-situ SEM observation. In the charging process, several ill-defined plateaus appear. Waves observed at over ca. 0.700 V (vs. Li(I)/Li) and the following wave at ca. 0.500e0.200 V are attributed to the solid electrolyte interface (SEI) layer formation [19] and the phase transition from crystalline Si (c-Si) to amorphous Li-Si alloy (a-LixSi) [34], respectively. Plateaus observed at the lower potentials should be due to the lithiation reaction forming a-LixSi to different Li-Si alloy phases, c-Li13Si4, and c-Li15Si4 [34,35]. These lithiation reaction seems to change at the following three potential regions; (I) over 0.200 V, (II) 0.200 / 0.070 V, (III) below 0.070 V (Fig. 2b). On the other hand, in the discharge process, only one plateau, which corresponds to the delithiation reaction from crystalline Li-Si alloy phases to a-LixSi, is recognized at ca. 0.250 / 0.550 V. In-situ SEM images of the Si electrode taken during the series of the electrochemical measurements are shown in Fig. 2c and d. As reported in our previous paper, each Si nanoparticle aggregate becomes larger with charging time and does not return to their original shapes even after discharge process [31]. We could not see a distinct damage of the Si nanoparticle in the process of charge and discharge. This irreversible behavior implies that dense packing of the Si aggregates is irreversibly loosened upon electrochemical cycling, as a plausible model shown in Fig. 3. Possibly it leads to the detachment of Si nanoparticle from the aggregates. In-situ SEM images of the Si nanoparticle aggregate electrode observed at different two places during first and second charge processes are indicated in Fig. 4. They were taken at the cut-off potential written under each image. In the first charge process, the volume of the Si nanoparticle aggregate dramatically changes at the potential from 0.150 to 0.110 V. At the second cycle, the volume change seems to initiate at more positive potential, 0.350e0.150 V. Recently we reported that the lithiation process for the Si expansion proceeds three-dimensionally and isotropically [33]. Based on this finding and the two dimensional expansion data estimated
Fig. 1. (a) Schematic drawing and (b) photograph of the newly-designed three-electrode type cell for in-situ SEM observation of battery reactions.
160
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
Fig. 2. (a) Charge-discharge curves recorded at the Si nanoparticle aggregate electrode in the in-situ three-electrode cell. (b) Enlarged data on the potential variation between 0.250e0.050 V vs. Li(I)/Li. The original data is shown in Fig. 2a. In-situ SEM images of the Si nanoparticle aggregate electrode during (c) first charge and (d) discharge processes. The electrolysis time is given in the bottom of each images. The electrolyte was a 1.0 M Li[TFSA]-[C2mim][FSA] IL. The charge-discharge process was conducted at 0.25C. The cut off potential for charge process was 0.050e2.000 V.
Fig. 3. Plausible model of electrode behavior for the Si nanoparticle aggregate electrode.
from Fig. 4, changes in the volume expansion rate as a function of cut-off potential are shown in Fig. 5. The expansion rate approaches the theoretical value (ca. 370%) for Li15Si4 alloy phase [36,37], as the potential becomes more negative potential than 0.070 V. It is noteworthy that at the second cycle, the expansion process initiates at more positive potential than the first cycle. Once a-LixSi phase is formed, in all the cycles that follow, lithiation process may readily proceed owing to the Li-ion conduction pathway formed in the Si aggregates during the first cycle. Taken together, in the first cycle, cLi15Si4 alloy is formed at very negative potential close to Li metal deposition and c-Li13Si4 alloy phase would be produced in the potential of 0.200 to 0.070 V, where the huge volume expansion occurs. In order to clarify the impact of the differences in the kind of electrolyte on morphology change and battery performance, we conducted the experiments under the same conditions except for use of a [Li(G4)][TFSA] electrolyte. Fig. 6 shows the charge-
discharge curves recorded in the [Li(G4)][TFSA]. The obtained curve is essentially the same as that obtained in the 1.0 M Li[TFSA][C2mim][FSA]. Much the same is true on the expansion behavior of Si aggregates (Fig. 7). However, as shown in Figs. 8 and 9, Li metal deposition was recognized at the cut-off potential of 0.001 V (vs. Li(I)/Li) in the [Li(G4)][TFSA], although it did not occur in the [C2mim][FSA] electrolyte. Black pillar or worm-like deposits, indicated by white arrows in Fig. 8, should be the Li deposits, because number of secondary electrons released from Li metal is fewer than other elements used in this experiment [38]. Often the Li deposit appeared on the Si nanoparticle aggregates (Fig. 9), since current would readily concentrate on the aggregates with rough surface but not on the smooth current collector. At this stage, we do not have a sufficient evidence that can explain the unexpected result on the different electrode behavior about Li deposition in these two electrolytes. We guess chemical composition of solid electrolyte interphase (SEI) formed in each electrolyte during the Li deposition process is involved in the difference. The SEI layer produced in the [Li(G4)][TFSA] would have a higher Li(I) conduction rate than that in 1.0 M Li[TFSA]-[C2mim] [FSA], i.e., a marginal difference in the overpotential for Li deposition would contribute to the difference. Further investigation is being conducted to reveal the reason using in-situ XPS analysis. 4. Conclusions In-situ SEM observation using IL electrolytes with their nonvolatile and antistatic nature is the effective analytical technique for creating the next-generation high-capacity battery. The use of Li metal reference electrode enabled the discussion on the electrode behavior of Si negative electrode with more precision compared to the previous similar papers that employed the two-electrode type cell. For example, the morphology variation in the Si nanoparticle aggregates, which relates to the phase transition of a-Si to c-Li13Si4 phase, occurs at 0.200 to 0.070 V (vs. Li(I)/Li) during the first charge process. At the second cycle, the variation initiates at more positive
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
161
Fig. 4. In-situ SEM images of the Si nanoparticle aggregate electrode during (a) first and (b) second charge processes. The cut-off potential (vs. Li(I)/Li.) is given in the bottom of each images. The electrolyte was a 1.0 M Li[TFSA]-[C2mim][FSA] IL. The charge-discharge process was conducted at constant current (0.1 C)-constant potential (2 h) (CC-CP) mode.
Fig. 5. Volume expansion rates estimated from the two-dimensional SEM images of the Si nanoparticle aggregate electrode during (a) first and (b) second charge processes at different cut-off potentials (vs. Li(I)/Li.). The electrolyte was a 1.0 M Li[TFSA]-[C2mim][FSA] IL. The charge-discharge process was conducted at CC (0.1 C)-CP (2 h) mode.
Fig. 6. (a) Charge-discharge curves recorded at the Si nanoparticle aggregate electrode in the in-situ three-electrode cell. (b) Enlarged data on the potential variation between 0.250e0.001 V (vs. Li(I)/Li). The original data is shown in Fig. 6a. The electrolyte was a [Li(G4)][TFSA] solvate IL. The charge-discharge process was conducted at 0.1 C. The cut off voltage for charge process was 0.001e2.000 V vs. Li(I)/Li.
162
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
Fig. 7. (a) In-situ SEM images of the Si nanoparticle aggregate electrode during first charge process. The cut-off potential (vs. Li(I)/Li) is given in the bottom of each images. The electrolyte was a [Li(G4)][TFSA] solvate IL. The charge-discharge process was conducted at CC (0.1 C)-CP (2 h) mode. (b) Enlarged SEM images at area a1 and a2 shown in Fig. 7a.
Fig. 8. In-situ SEM images of the Si nanoparticle aggregate electrode after first charge process at the cut-off potential of 0.001 V (vs. Li(I)/Li). The electrolyte was a [Li(G4)][TFSA] solvate IL. The charge-discharge process was conducted at CP (0.5 h) mode. These images were taken at different places.
Fig. 9. In-situ SEM images of the Si nanoparticle aggregate electrode (a) before and (b) after first charge process at the cut-off potential of 0.001 V (vs. Li(I)/Li). The electrolyte was a [Li(G4)][TFSA] solvate IL. The charge-discharge process was conducted at CP (0.5 h) mode. (c) An enlarged SEM image of the red circle part shown in Fig. 9b.
potential. We revealed that these morphology variations have a strong correlation with the charge-discharge curves observed. The advantage of the in-situ SEM observation cell with the Li(I)/Li reference electrode will be widely recognized as one of the major techniques to analyze the battery reactions in the IL electrolytes. Acknowledgments Part of this research was supported by Grants-in-Aid for Scientific Research (grant numbers 15H03591, 15K13287, 15H02202, 16K14539, and 19H02814) from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and by the Advanced Low Carbon Technology Research and Development Program (ALCA) for Specially Promoted Research for Innovative
Next Generation Batteries (SPRING), the Japan Science and Technology Agency (JST) and JST-MIRAI program. References [1] S. Zhang, K. Ueno, K. Dokko, M. Watanabe, Recent advances in electrolytes for lithiumesulfur batteries, Adv. Energy Mater. 5 (2015) 1500117. [2] J. Balach, J. Linnemann, T. Jaumann, Lars Giebeler, Metal-based nanostructured materials for advanced lithiumesulfur batteries, J. Mater. Chem. A 6 (2018) 23127e23168. [3] L. Zhang, Y. Wang, Z. Niu, J. Chen, Advanced nanostructured carbon-based materials for rechargeable lithium-sulfur batteries, Carbon 141 (2019) 400e416. [4] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, High-power all-solid-state batteries using sulfide superionic conductors, Nat. Energy 1 (2016) 16030. [5] Y. Kato, S. Shiotani, K. Morita, K. Suzuki, M. Hirayama, R. Kanno, All-solid-state
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
[6]
[7]
[8]
[9] [10]
[11]
[12]
[13]
[14]
[15]
[16] [17] [18] [19]
[20]
[21] [22] [23]
batteries with thick electrode configurations, J. Phys. Chem. Lett. 9 (2018) 607e613. H. Tsukasaki, Y. Mori, M. Otoyama, S. Yubuchi, T. Asano, Y. Tanaka, T. Ohno, S. Mori, A. Hayashi, M. Tatsumisago, Crystallization behavior of the Li2SeP2S5 glass electrolyte in the LiNi1/3Mn1/3Co1/3O2 positive electrode layer, Sci. Rep. 8 (2018) 6214. M. Salama, R. Attias, B. Hirsch, R. Yemini, Y. Gofer, M. Noked, D. Aurbach, On the feasibility of practical MgS batteries: practical limitations associated with metallic magnesium anodes, ACS Appl. Mater. Interfaces 10 (2018) 36910e36917. K. Shimokawa, H. Matsumoto, T. Ichitsubo, Solvation-structure modification by concentrating Mg(TFSA)2MgCl2Triglyme ternary electrolyte, J. Phys. Chem. Lett. 9 (2018) 4732e4737. M. Mao, T. Gao, S. Houa, C. Wang, A critical review of cathodes for rechargeable Mg batteries, Chem. Soc. Rev. 47 (2018) 8804e8841. T. Tsuda, I. Kokubo, M. Kawabata, M. Yamagata, M. Ishikawa, S. Kusumoto, A. Imanishi, S. Kuwabata, Electrochemical energy storage device with a lewis acidic AlBr31-Ethyl-3-methylimidazolium bromide room-temperature ionic liquid, J. Electrochem. Soc. 161 (2014) A908eA914. M.-C. Lin, M. Gong, B. Lu, Y. Wu, D.-Y. Wang, M. Guan, M. Angell, C. Chen, J. Yang, B.-J. Hwang, H. Dai, Ultrafast rechargeable aluminum-ion battery, Nature 520 (2015) 324e328. M. Angell, C.-J. Pan, Y. Rong, C. Yuan, M.-C. Lin, B.-J. Hwang, H. Dai, High Coulombic efficiency aluminum-ion battery using an AlCl3-urea ionic liquid analog electrolyte, Proc. Natl. Acad. Sci. U.S.A. 114 (2017) 834e839. C.-Y. Chen, T. Tsuda, S. Kuwabata, C.L. Hussey, Rechargeable aluminum batteries utilizing a chloroaluminate inorganic ionic liquid electrolyte, Chem. Commun. 54 (2018) 4164e4167. T. Tsuda, G.R. Stafford, C.L. Hussey, ReviewElectrochemical surface finishing and energy storage Technology with room-temperature haloaluminate ionic liquids and mixtures, J. Electrochem. Soc. 164 (2017) H5007eH5017. K. Nishikawa, T. Mori, T. Nishida, Y. Fukunaka, M. Rosso, Li dendrite growth and Liþ ionic mass transfer phenomenon, J. Electroanal. Chem. 661 (2011) 84e89. K. Nishikawa, H. Munakata, K. Kanamura, In-situ observation of one silicon particle during the first charging, J. Power Sources 243 (2013) 630e634. P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, second ed., Wiley-VCH, Weinheim, 2008. H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, second ed., WileyInterscience, New Jersey, 2011. H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, Fast cycling of Li/LiCoO2 cell with low-viscosity ionic liquids based on bis(fluorosulfonyl)imide [FSI], J. Power Sources 160 (2006) 1308e1313. M. Watanabe, M.L. Thomas, S. Zhang, K. Ueno, T. Yasuda, K. Dokko, Application of ionic liquids to energy storage and conversion materials and devices, Chem. Rev. 117 (2017) 7190e7239. D.N. Qian, C. Ma, K.L. More, Y.S. Meng, M.F. Chi, Advanced analytical electron microscopy for lithium-ion batteries, NPG Asia Mater. 7 (2015) e193. C.P. Grey, J.M. Tarascon, Sustainability and in situ monitoring in battery development, Nat. Mater. 16 (2017) 45e56. S. Arimoto, H. Kageyama, T. Torimoto, S. Kuwabata, Development of in situ
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36] [37] [38]
163
scanning electron microscope system for real time observation of metal deposition from ionic liquid, Electrochem. Commun. 10 (2008) 1901e1904. T. Uematsu, J.-T. Han, T. Tsuda, S. Kuwabata, Metal-ion diffusion in ionic liquid studied by electrochemical scanning electron microscopy with X-ray fluorescence spectrometry, J. Phys. Chem. C 116 (2012) 20902e20907. Y.-T. Hsieh, T. Tsuda, S. Kuwabata, SEM as a facile tool for real-time monitoring of microcrystal growth during electrodeposition: the merit of ionic liquids, Anal. Chem. 89 (2017) 7249e7254. €nig, In situ scanning electron D. Chena, S. Indris, M. Schulz, B. Gamer, R. Mo microscopy on lithium-ion battery electrodes using an ionic liquid, J. Power Sources 196 (2011) 6382e6387. E. Strelcov, J. Cothren, D. Leonard, A.Y. Borisevich, A. Kolmakov, In situ SEM study of lithium intercalation in individual V2O5 nanowires, Nanoscale 7 (2015) 3022e3027. X. Xia, C.V. Di Leo, X.W. Gu, J.R. Greer, In situ LithiationDelithiation of mechanically robust CuSi CoreShell nanolattices in a scanning electron microscope, ACS Energy Lett. 1 (2016) 492e499. X. Zhou, T. Li, Y. Cui, Y. Fu, Y. Liu, L. Zhu, In situ focused ion beam scanning electron microscope study of microstructural evolution of single tin particle anode for Li-ion batteries, ACS Appl. Mater. Interfaces 11 (2019) 1733e1738. T. Tsuda, T. Kanetsuku, T. Sano, Y. Oshima, K. Ui, M. Yamagata, M. Ishikawa, S. Kuwabata, In situ SEM observation of the Si negative electrode reaction in an ionic-liquid-based lithium-ion secondary battery, Microscopy 64 (2015) 159e168. C.-Y. Chen, T. Sano, T. Tsuda, K. Ui, Y. Oshima, M. Yamagata, M. Ishikawa, M. Haruta, T. Doi, M. Inaba, In situ scanning electron microscopy of silicon anode reactions in lithium-ion batteries during charge/discharge processes, Sci. Rep. 6 (2016) 36153. C.-Y. Chen, A. Sawamura, T. Tsuda, S. Uchida, M. Ishikawa, S. Kuwabata, Visualization of Si anode reactions in coin-type cells via operando scanning electron microscopy, ACS Appl. Mater. Interfaces 9 (2017) 35511e35515. T. Tsuda, A. Imanishi, T. Sano, A. Sawamura, T. Kamidaira, C.-Y. Chen, S. Uchida, S. Kusumoto, M. Ishikawa, S. Kuwabata, In situ electron microscopy and X-ray photoelectron spectroscopy for high capacity anodes in next-generation ionic liquid-based Li batteries, Electrochim. Acta 279 (2018) 136e142. M.J. Loveridge, M.J. Lain, I.D. Johnson, A. Roberts, S.D. Beattie, R. Dashwood, J.A. Darr, R. Bhagat, Towards high capacity Li-ion batteries based on silicongraphene composite anodes and sub-micron V-doped LiFePO4 cathodes, Sci. Rep. 6 (2016) 37787. B. Key, M. Morcrette, J.-M. Tarascon, C.P. Grey, Pair distribution function analysis and solid state NMR studies of silicon electrodes for lithium ion batteries: understanding the (De)lithiation mechanisms, J. Am. Chem. Soc. 133 (2011) 503e512. B.A. Boukamp, G.C. Lesh, R.A. Huggins, All-solid lithium electrodes with mixed-conductor matrix, J. Electrochem. Soc. 128 (1981) 725e729. H. Kim, C.Y. Chou, J.G. Ekerdt, G.S. Hwang, Structure and properties of Li-Si alloys: a first-principles study, J. Phys. Chem. C 115 (2011) 2514e2521. J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer, J. Michael, Scanning Electron Microscopy and X-Ray Microanalysis, third ed., Springer þ Business Media, New York, 2003.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In-situ scanning electron microscope observation of electrode reactions related to battery material Tetsuya Tsuda**, Kei Hosoya, Teruki Sano, Susumu Kuwabata* Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2019 Received in revised form 23 June 2019 Accepted 27 June 2019 Available online 28 June 2019
Scanning electron microscopy (SEM) for the electrode reaction in next-generation battery with ionic liquid (IL) electrolyte, which has both high stability under vacuum and an antistatic nature, is applied to the binder-free high capacity Si anode to observe the morphology change of the Si nanoparticle aggregate active material at different potentials during charge-discharge processes. In the IL electrolyte, 1-ethyl-3-methylimidazolium bis(fluoromethylsulfonyl)amide ([C2mim][FSA]) with 1.0 M lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), the morphology greatly varied at the plateau observed at 0.200 to 0.070 V (vs. Li(I)/Li) in the charge-discharge curve during the first cycle, suggesting that phase transition of a-Si to c-Li13Si4 is a potential problematic process. After the first cycle, the lithiation process initiated at more positive potential. Similar behavior was also recognized in the Li[TFSA]‒tetraglyme solvate IL electrolyte in a 1:1 molar ratio ([Li(G4)][TFSA]). © 2019 Published by Elsevier Ltd.
Keywords: In situ Ionic liquid Battery Si anode Scanning electron microscopy
1. Introduction Several requirements to future battery systems, such as higher capacity and quicker charging, will give serious issues, e.g., morphology change of the electrodes, variation in electrode|electrolyte interphase and undesirable cell deformation. Obviously these issues lead to the deterioration of battery. Although there are some promising future batteries including Li-S rechargeable battery [1e3], all solid Li rechargeable battery [4e6], Mg metal rechargeable battery [7e9] and Al metal rechargeable battery [10e14], these batteries have the above mentioned issues that should be overcome. It means that direct analysis or in-situ observation of the battery reactions will be a key technique to improve the battery performance. In-situ optical microscope observations using airtight electrochemical cells have already been reported to date [15,16]. What is worth noting is that most future batteries employ nonvolatile electrolytes, solid electrolyte and ionic liquid (IL) electrolyte, suggesting that electron microscopy, which requires vacuum condition, is applicable to the battery reaction observation without such airtight cells. ILs are a subset of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Tsuda), kuwabata@chem. eng.osaka-u.ac.jp (S. Kuwabata). https://doi.org/10.1016/j.electacta.2019.06.165 0013-4686/© 2019 Published by Elsevier Ltd.
anhydrous liquid salts that contain only cationic and anionic species and can maintain a liquid state below 373 K. The common features of ILs are negligible vapor pressure, flame resistance, wide electrochemical window and relatively-high ionic conductivity [17,18]. These features are controllable by the combination of cationic and anionic species in the ILs. It is then quite natural to design the IL electrolytes for next-generation batteries by controlling the ionic combination [17e20]. Besides, using IL electrolytes with a negligible vapor pressure enable in-situ and operando analytical techniques conducted along with the electrochemical measurements under vacuum condition. Direct observation of the morphology variations in nextgeneration batteries by in-situ and operando analytical techniques is increasingly important to get new leads for improving the battery performance [21,22]. Several research groups including us have attempted to establish scanning electron microscopy (SEM) techniques for the in-situ and operando observation of electrode reactions in the specially designed electrochemical cells with the IL electrolytes [23e29]. Recently morphology variations in several Si anodes in the full cell type Li-ion battery, which can give useful actual information during the battery operation compared to the half cell type one, have successfully been observed by similar in situ SEM techniques [30e33]. However, there is no detailed information on the variations in the Si anode at different potentials during the charge-discharge processes. In this article, we reveal the morphology variation of the binder free Si nanoparticle aggregate
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
anode by SEM observation at different potentials. For this purpose, the specially designed three-electrode type Li-ion batteries were fabricated using two different IL electrolytes, 1-ethyl-3methylimidazolium bis(fluoromethylsulfonyl)amide ([C2mim] [FSA]) IL with 1.0 M lithium bis(trifluoromethanesulfonyl)amide (Li [TFSA]) and Li[TFSA]‒tetraglyme solvate IL in a 1:1 molar ratio ([Li(G4)][TFSA]). 2. Experimental section In situ SEM observation of the binder-free Si anodes was performed using a three-electrode type full cell with a mesh-type anode, a LiCoO2 composite cathode with an Al foil current collector (3.0 mAh cm2, Hohsen (Japan)), a Ni wire (f 0.2 mm, Nilaco (Japan)), and two sheets of glass microfiber filter separators (GF/A, Whatman (UK)). The Ni wire sandwiched between two glass microfiber filter separators was used as a reference electrode after Li metal electrodeposition on the Ni wire electrode by using LiCoO2 cathode as a counter electrode. The Li metal deposition was conducted at -0.2 mA for 1 h. Its potential was calibrated to LiCoO2 after electrochemical measurements. The theoretical LiCoO2 cathode capacity was enough to observe the Si anode reaction. Binder-free handmade Si anodes were fabricated by an electrophoretic deposition (EPD) method similar to previous paper, but not using acetylene black and citric acid in this study [31]. The solution used for the binder-free Si anode preparation was dry acetonitrile containing 1.0 g L1 of Si nanoparticle aggregates (325 mesh, Rare Metallic Co., LTD. (Japan)). A platinum plate was used as a cathode and a copper mesh was served as an anode. The electrodes were separated by 2 cm in the solution and a voltage of 100 V was kept between them with duration of 60 s. A schematic drawing and photograph of the electrochemical cell for in-situ SEM observation of the Si anode are depicted in Fig. 1. The electrolytes were [C2mim][FSA] (Kanto Chemical Co., Inc. (Japan)) with 1.0 M Li[TFSA] (Morita Chemical Industries Co., Ltd. (Japan)) and [Li(G4)][TFSA]. All Li-ion batteries for in-situ experiments were assembled in an argon-filled glove box (Vacuum Atmospheres Company (USA), OMNI-LAB; H2O, O2 < 1 ppm). Electrochemical in-situ SEM observation system was fabricated by attaching a feed-through terminal into commercially available SEM systems (VE-8800, KEYENCE (Japan); S-3400N, Hitachi (Japan)) to examine the electrode reaction. Electrochemical experiments were controlled with a potentiostat/galvanostat (VersaSTAT 4, Princeton Applied Research (USA)). 3. Results and discussion We have already succeeded in the in-situ SEM observation of
159
different types of Si active materials and their composite negative electrodes for next-generation high capacity battery and revealed that the behavior of the morphology variation is closely related to the battery performance [31,32]. Si anodes with a favorable performance show a good reversible morphology alteration during the charge-discharge process, suggesting that it is important to design the electrode configuration considering the morphology variation after charge and discharge in order to fabricate high performance negative electrodes. However, no detailed information on the relevance between morphology change and charge-discharge potential of Si negative electrode has not yet been reported. Fig. 2a shows charge-discharge curves at the first cycle of the binder-free Si nanoparticle aggregate negative electrode, which were taken by a constant current mode at 0.25C in the cell for electrochemical in-situ SEM observation. In the charging process, several ill-defined plateaus appear. Waves observed at over ca. 0.700 V (vs. Li(I)/Li) and the following wave at ca. 0.500e0.200 V are attributed to the solid electrolyte interface (SEI) layer formation [19] and the phase transition from crystalline Si (c-Si) to amorphous Li-Si alloy (a-LixSi) [34], respectively. Plateaus observed at the lower potentials should be due to the lithiation reaction forming a-LixSi to different Li-Si alloy phases, c-Li13Si4, and c-Li15Si4 [34,35]. These lithiation reaction seems to change at the following three potential regions; (I) over 0.200 V, (II) 0.200 / 0.070 V, (III) below 0.070 V (Fig. 2b). On the other hand, in the discharge process, only one plateau, which corresponds to the delithiation reaction from crystalline Li-Si alloy phases to a-LixSi, is recognized at ca. 0.250 / 0.550 V. In-situ SEM images of the Si electrode taken during the series of the electrochemical measurements are shown in Fig. 2c and d. As reported in our previous paper, each Si nanoparticle aggregate becomes larger with charging time and does not return to their original shapes even after discharge process [31]. We could not see a distinct damage of the Si nanoparticle in the process of charge and discharge. This irreversible behavior implies that dense packing of the Si aggregates is irreversibly loosened upon electrochemical cycling, as a plausible model shown in Fig. 3. Possibly it leads to the detachment of Si nanoparticle from the aggregates. In-situ SEM images of the Si nanoparticle aggregate electrode observed at different two places during first and second charge processes are indicated in Fig. 4. They were taken at the cut-off potential written under each image. In the first charge process, the volume of the Si nanoparticle aggregate dramatically changes at the potential from 0.150 to 0.110 V. At the second cycle, the volume change seems to initiate at more positive potential, 0.350e0.150 V. Recently we reported that the lithiation process for the Si expansion proceeds three-dimensionally and isotropically [33]. Based on this finding and the two dimensional expansion data estimated
Fig. 1. (a) Schematic drawing and (b) photograph of the newly-designed three-electrode type cell for in-situ SEM observation of battery reactions.
160
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
Fig. 2. (a) Charge-discharge curves recorded at the Si nanoparticle aggregate electrode in the in-situ three-electrode cell. (b) Enlarged data on the potential variation between 0.250e0.050 V vs. Li(I)/Li. The original data is shown in Fig. 2a. In-situ SEM images of the Si nanoparticle aggregate electrode during (c) first charge and (d) discharge processes. The electrolysis time is given in the bottom of each images. The electrolyte was a 1.0 M Li[TFSA]-[C2mim][FSA] IL. The charge-discharge process was conducted at 0.25C. The cut off potential for charge process was 0.050e2.000 V.
Fig. 3. Plausible model of electrode behavior for the Si nanoparticle aggregate electrode.
from Fig. 4, changes in the volume expansion rate as a function of cut-off potential are shown in Fig. 5. The expansion rate approaches the theoretical value (ca. 370%) for Li15Si4 alloy phase [36,37], as the potential becomes more negative potential than 0.070 V. It is noteworthy that at the second cycle, the expansion process initiates at more positive potential than the first cycle. Once a-LixSi phase is formed, in all the cycles that follow, lithiation process may readily proceed owing to the Li-ion conduction pathway formed in the Si aggregates during the first cycle. Taken together, in the first cycle, cLi15Si4 alloy is formed at very negative potential close to Li metal deposition and c-Li13Si4 alloy phase would be produced in the potential of 0.200 to 0.070 V, where the huge volume expansion occurs. In order to clarify the impact of the differences in the kind of electrolyte on morphology change and battery performance, we conducted the experiments under the same conditions except for use of a [Li(G4)][TFSA] electrolyte. Fig. 6 shows the charge-
discharge curves recorded in the [Li(G4)][TFSA]. The obtained curve is essentially the same as that obtained in the 1.0 M Li[TFSA][C2mim][FSA]. Much the same is true on the expansion behavior of Si aggregates (Fig. 7). However, as shown in Figs. 8 and 9, Li metal deposition was recognized at the cut-off potential of 0.001 V (vs. Li(I)/Li) in the [Li(G4)][TFSA], although it did not occur in the [C2mim][FSA] electrolyte. Black pillar or worm-like deposits, indicated by white arrows in Fig. 8, should be the Li deposits, because number of secondary electrons released from Li metal is fewer than other elements used in this experiment [38]. Often the Li deposit appeared on the Si nanoparticle aggregates (Fig. 9), since current would readily concentrate on the aggregates with rough surface but not on the smooth current collector. At this stage, we do not have a sufficient evidence that can explain the unexpected result on the different electrode behavior about Li deposition in these two electrolytes. We guess chemical composition of solid electrolyte interphase (SEI) formed in each electrolyte during the Li deposition process is involved in the difference. The SEI layer produced in the [Li(G4)][TFSA] would have a higher Li(I) conduction rate than that in 1.0 M Li[TFSA]-[C2mim] [FSA], i.e., a marginal difference in the overpotential for Li deposition would contribute to the difference. Further investigation is being conducted to reveal the reason using in-situ XPS analysis. 4. Conclusions In-situ SEM observation using IL electrolytes with their nonvolatile and antistatic nature is the effective analytical technique for creating the next-generation high-capacity battery. The use of Li metal reference electrode enabled the discussion on the electrode behavior of Si negative electrode with more precision compared to the previous similar papers that employed the two-electrode type cell. For example, the morphology variation in the Si nanoparticle aggregates, which relates to the phase transition of a-Si to c-Li13Si4 phase, occurs at 0.200 to 0.070 V (vs. Li(I)/Li) during the first charge process. At the second cycle, the variation initiates at more positive
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
161
Fig. 4. In-situ SEM images of the Si nanoparticle aggregate electrode during (a) first and (b) second charge processes. The cut-off potential (vs. Li(I)/Li.) is given in the bottom of each images. The electrolyte was a 1.0 M Li[TFSA]-[C2mim][FSA] IL. The charge-discharge process was conducted at constant current (0.1 C)-constant potential (2 h) (CC-CP) mode.
Fig. 5. Volume expansion rates estimated from the two-dimensional SEM images of the Si nanoparticle aggregate electrode during (a) first and (b) second charge processes at different cut-off potentials (vs. Li(I)/Li.). The electrolyte was a 1.0 M Li[TFSA]-[C2mim][FSA] IL. The charge-discharge process was conducted at CC (0.1 C)-CP (2 h) mode.
Fig. 6. (a) Charge-discharge curves recorded at the Si nanoparticle aggregate electrode in the in-situ three-electrode cell. (b) Enlarged data on the potential variation between 0.250e0.001 V (vs. Li(I)/Li). The original data is shown in Fig. 6a. The electrolyte was a [Li(G4)][TFSA] solvate IL. The charge-discharge process was conducted at 0.1 C. The cut off voltage for charge process was 0.001e2.000 V vs. Li(I)/Li.
162
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
Fig. 7. (a) In-situ SEM images of the Si nanoparticle aggregate electrode during first charge process. The cut-off potential (vs. Li(I)/Li) is given in the bottom of each images. The electrolyte was a [Li(G4)][TFSA] solvate IL. The charge-discharge process was conducted at CC (0.1 C)-CP (2 h) mode. (b) Enlarged SEM images at area a1 and a2 shown in Fig. 7a.
Fig. 8. In-situ SEM images of the Si nanoparticle aggregate electrode after first charge process at the cut-off potential of 0.001 V (vs. Li(I)/Li). The electrolyte was a [Li(G4)][TFSA] solvate IL. The charge-discharge process was conducted at CP (0.5 h) mode. These images were taken at different places.
Fig. 9. In-situ SEM images of the Si nanoparticle aggregate electrode (a) before and (b) after first charge process at the cut-off potential of 0.001 V (vs. Li(I)/Li). The electrolyte was a [Li(G4)][TFSA] solvate IL. The charge-discharge process was conducted at CP (0.5 h) mode. (c) An enlarged SEM image of the red circle part shown in Fig. 9b.
potential. We revealed that these morphology variations have a strong correlation with the charge-discharge curves observed. The advantage of the in-situ SEM observation cell with the Li(I)/Li reference electrode will be widely recognized as one of the major techniques to analyze the battery reactions in the IL electrolytes. Acknowledgments Part of this research was supported by Grants-in-Aid for Scientific Research (grant numbers 15H03591, 15K13287, 15H02202, 16K14539, and 19H02814) from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and by the Advanced Low Carbon Technology Research and Development Program (ALCA) for Specially Promoted Research for Innovative
Next Generation Batteries (SPRING), the Japan Science and Technology Agency (JST) and JST-MIRAI program. References [1] S. Zhang, K. Ueno, K. Dokko, M. Watanabe, Recent advances in electrolytes for lithiumesulfur batteries, Adv. Energy Mater. 5 (2015) 1500117. [2] J. Balach, J. Linnemann, T. Jaumann, Lars Giebeler, Metal-based nanostructured materials for advanced lithiumesulfur batteries, J. Mater. Chem. A 6 (2018) 23127e23168. [3] L. Zhang, Y. Wang, Z. Niu, J. Chen, Advanced nanostructured carbon-based materials for rechargeable lithium-sulfur batteries, Carbon 141 (2019) 400e416. [4] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, High-power all-solid-state batteries using sulfide superionic conductors, Nat. Energy 1 (2016) 16030. [5] Y. Kato, S. Shiotani, K. Morita, K. Suzuki, M. Hirayama, R. Kanno, All-solid-state
T. Tsuda et al. / Electrochimica Acta 319 (2019) 158e163
[6]
[7]
[8]
[9] [10]
[11]
[12]
[13]
[14]
[15]
[16] [17] [18] [19]
[20]
[21] [22] [23]
batteries with thick electrode configurations, J. Phys. Chem. Lett. 9 (2018) 607e613. H. Tsukasaki, Y. Mori, M. Otoyama, S. Yubuchi, T. Asano, Y. Tanaka, T. Ohno, S. Mori, A. Hayashi, M. Tatsumisago, Crystallization behavior of the Li2SeP2S5 glass electrolyte in the LiNi1/3Mn1/3Co1/3O2 positive electrode layer, Sci. Rep. 8 (2018) 6214. M. Salama, R. Attias, B. Hirsch, R. Yemini, Y. Gofer, M. Noked, D. Aurbach, On the feasibility of practical MgS batteries: practical limitations associated with metallic magnesium anodes, ACS Appl. Mater. Interfaces 10 (2018) 36910e36917. K. Shimokawa, H. Matsumoto, T. Ichitsubo, Solvation-structure modification by concentrating Mg(TFSA)2MgCl2Triglyme ternary electrolyte, J. Phys. Chem. Lett. 9 (2018) 4732e4737. M. Mao, T. Gao, S. Houa, C. Wang, A critical review of cathodes for rechargeable Mg batteries, Chem. Soc. Rev. 47 (2018) 8804e8841. T. Tsuda, I. Kokubo, M. Kawabata, M. Yamagata, M. Ishikawa, S. Kusumoto, A. Imanishi, S. Kuwabata, Electrochemical energy storage device with a lewis acidic AlBr31-Ethyl-3-methylimidazolium bromide room-temperature ionic liquid, J. Electrochem. Soc. 161 (2014) A908eA914. M.-C. Lin, M. Gong, B. Lu, Y. Wu, D.-Y. Wang, M. Guan, M. Angell, C. Chen, J. Yang, B.-J. Hwang, H. Dai, Ultrafast rechargeable aluminum-ion battery, Nature 520 (2015) 324e328. M. Angell, C.-J. Pan, Y. Rong, C. Yuan, M.-C. Lin, B.-J. Hwang, H. Dai, High Coulombic efficiency aluminum-ion battery using an AlCl3-urea ionic liquid analog electrolyte, Proc. Natl. Acad. Sci. U.S.A. 114 (2017) 834e839. C.-Y. Chen, T. Tsuda, S. Kuwabata, C.L. Hussey, Rechargeable aluminum batteries utilizing a chloroaluminate inorganic ionic liquid electrolyte, Chem. Commun. 54 (2018) 4164e4167. T. Tsuda, G.R. Stafford, C.L. Hussey, ReviewElectrochemical surface finishing and energy storage Technology with room-temperature haloaluminate ionic liquids and mixtures, J. Electrochem. Soc. 164 (2017) H5007eH5017. K. Nishikawa, T. Mori, T. Nishida, Y. Fukunaka, M. Rosso, Li dendrite growth and Liþ ionic mass transfer phenomenon, J. Electroanal. Chem. 661 (2011) 84e89. K. Nishikawa, H. Munakata, K. Kanamura, In-situ observation of one silicon particle during the first charging, J. Power Sources 243 (2013) 630e634. P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, second ed., Wiley-VCH, Weinheim, 2008. H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, second ed., WileyInterscience, New Jersey, 2011. H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, Fast cycling of Li/LiCoO2 cell with low-viscosity ionic liquids based on bis(fluorosulfonyl)imide [FSI], J. Power Sources 160 (2006) 1308e1313. M. Watanabe, M.L. Thomas, S. Zhang, K. Ueno, T. Yasuda, K. Dokko, Application of ionic liquids to energy storage and conversion materials and devices, Chem. Rev. 117 (2017) 7190e7239. D.N. Qian, C. Ma, K.L. More, Y.S. Meng, M.F. Chi, Advanced analytical electron microscopy for lithium-ion batteries, NPG Asia Mater. 7 (2015) e193. C.P. Grey, J.M. Tarascon, Sustainability and in situ monitoring in battery development, Nat. Mater. 16 (2017) 45e56. S. Arimoto, H. Kageyama, T. Torimoto, S. Kuwabata, Development of in situ
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36] [37] [38]
163
scanning electron microscope system for real time observation of metal deposition from ionic liquid, Electrochem. Commun. 10 (2008) 1901e1904. T. Uematsu, J.-T. Han, T. Tsuda, S. Kuwabata, Metal-ion diffusion in ionic liquid studied by electrochemical scanning electron microscopy with X-ray fluorescence spectrometry, J. Phys. Chem. C 116 (2012) 20902e20907. Y.-T. Hsieh, T. Tsuda, S. Kuwabata, SEM as a facile tool for real-time monitoring of microcrystal growth during electrodeposition: the merit of ionic liquids, Anal. Chem. 89 (2017) 7249e7254. €nig, In situ scanning electron D. Chena, S. Indris, M. Schulz, B. Gamer, R. Mo microscopy on lithium-ion battery electrodes using an ionic liquid, J. Power Sources 196 (2011) 6382e6387. E. Strelcov, J. Cothren, D. Leonard, A.Y. Borisevich, A. Kolmakov, In situ SEM study of lithium intercalation in individual V2O5 nanowires, Nanoscale 7 (2015) 3022e3027. X. Xia, C.V. Di Leo, X.W. Gu, J.R. Greer, In situ LithiationDelithiation of mechanically robust CuSi CoreShell nanolattices in a scanning electron microscope, ACS Energy Lett. 1 (2016) 492e499. X. Zhou, T. Li, Y. Cui, Y. Fu, Y. Liu, L. Zhu, In situ focused ion beam scanning electron microscope study of microstructural evolution of single tin particle anode for Li-ion batteries, ACS Appl. Mater. Interfaces 11 (2019) 1733e1738. T. Tsuda, T. Kanetsuku, T. Sano, Y. Oshima, K. Ui, M. Yamagata, M. Ishikawa, S. Kuwabata, In situ SEM observation of the Si negative electrode reaction in an ionic-liquid-based lithium-ion secondary battery, Microscopy 64 (2015) 159e168. C.-Y. Chen, T. Sano, T. Tsuda, K. Ui, Y. Oshima, M. Yamagata, M. Ishikawa, M. Haruta, T. Doi, M. Inaba, In situ scanning electron microscopy of silicon anode reactions in lithium-ion batteries during charge/discharge processes, Sci. Rep. 6 (2016) 36153. C.-Y. Chen, A. Sawamura, T. Tsuda, S. Uchida, M. Ishikawa, S. Kuwabata, Visualization of Si anode reactions in coin-type cells via operando scanning electron microscopy, ACS Appl. Mater. Interfaces 9 (2017) 35511e35515. T. Tsuda, A. Imanishi, T. Sano, A. Sawamura, T. Kamidaira, C.-Y. Chen, S. Uchida, S. Kusumoto, M. Ishikawa, S. Kuwabata, In situ electron microscopy and X-ray photoelectron spectroscopy for high capacity anodes in next-generation ionic liquid-based Li batteries, Electrochim. Acta 279 (2018) 136e142. M.J. Loveridge, M.J. Lain, I.D. Johnson, A. Roberts, S.D. Beattie, R. Dashwood, J.A. Darr, R. Bhagat, Towards high capacity Li-ion batteries based on silicongraphene composite anodes and sub-micron V-doped LiFePO4 cathodes, Sci. Rep. 6 (2016) 37787. B. Key, M. Morcrette, J.-M. Tarascon, C.P. Grey, Pair distribution function analysis and solid state NMR studies of silicon electrodes for lithium ion batteries: understanding the (De)lithiation mechanisms, J. Am. Chem. Soc. 133 (2011) 503e512. B.A. Boukamp, G.C. Lesh, R.A. Huggins, All-solid lithium electrodes with mixed-conductor matrix, J. Electrochem. Soc. 128 (1981) 725e729. H. Kim, C.Y. Chou, J.G. Ekerdt, G.S. Hwang, Structure and properties of Li-Si alloys: a first-principles study, J. Phys. Chem. C 115 (2011) 2514e2521. J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer, J. Michael, Scanning Electron Microscopy and X-Ray Microanalysis, third ed., Springer þ Business Media, New York, 2003.